1. Field of the Invention
The present invention relates to an automatic tuning apparatus. More specifically, the present invention relates to an improved automatic tuning apparatus employing a voltage controlled variable reactance device.
2. Description of the Prior Art
Recently an automatic tuning apparatus employing a voltage controlled variable capacitor has been widely used in radio receivers, television receivers and the like. Such a voltage controlled variable capacitor uses a capacitance formed at a barrier portion of a diode, variable as a function of a reverse voltage applied thereacross and is known as a variable capacitance diode. In such an automatic tuning apparatus, it is necessary to provide a voltage supply capable of providing a voltage variable as chosen in response to manual operation, which causes a capacitance with which the tuning apparatus selects a desired tuning frequency and thus selects a desired transmitting station. It is well known that such a voltage is closely related with the tuning frequency and thus the selected transmitting station.
One type of such a voltage supply for providing a voltage variable in response to manual operation to a variable capacitance diode comprises a capacitor and a charging/discharging control circuit therefor, a voltage across the capacitor, as selected in response to a tuning output, being applied to the variable capacitance diode. More specifically, the capacitor is charged or discharged by the charging/discharging control circuit in response to manual operation until a voltage across the capacitor causes capacitance in the diode with which the tuning circuit tunes to a given frequency to provide a tuning output therefrom when the charging/discharging control circuit is disabled to discontinue to charge or discharge the capacitor, so that the voltage thus provided across the capacitor remains the same. If another tuning frequency is desired, the charging/discharging control circuit is again enabled through manual operation and the abovementioned operation is repeated until another channel is selected.
A disadvantage encountered in the automatic tuning apparatus with a voltage controlled variable capacitance diode supplied with a control voltage from a capacitor is that the voltage across the capacitor gradually becomes lower because of leakage through an undesired leakage path, which causes detuning of the selecting apparatus and thus results in a poor quality of sound by the speaker of the radio receiver. In order to solve such a problem it is required to provide a circuit for compensating the decrease in voltage across the capacitor because of leakage, which makes the circuit complicated and the system expensive. Another disadvantage in the automatic tuning apparatus using a variable capacitance diode supplied with a voltage across the capacitor is that the voltage across the capacitor is charged or discharged in an exponential manner rather than in a linear manner, which makes it difficult to achieve precise automatic tuning. It is desired that an improved voltage supply source for providing a variable voltage to a variable capacitance diode is provided, which is suited for employment in an automatic tuning apparatus.
A voltage storing device of interest in connection with the present invention is disclosed in U.S. Pat. No. 3,753,110 issued Aug. 14, 1973 to Hironosuke Ikeda et al, and assigned to Sanyo Electric Co., Ltd, the same assignee as that of the present invention. As set forth in the referenced patent, Professor Takehiko Takahashi and Assistant Professor Osamu Yamamoto, Technological Department of Nagoya University, announced their study on the electrochemical potential memory device by the use of a solid state electrolyte at the 22nd annual assembly of Japan Chemical Association held on Apr. 5th to 7th, 1969. Briefly stated, this device comprises an Ag electrode as a cathode, an Ag-Te alloy electrode as an anode, and a solid state electrolyte having high ion conductivity, such as RbAg.sub.4 I.sub.5 sandwiched between both electrodes. When a DC voltages is applied to the device so that the Ag electrode may be negative, a portion of Ag contained in the Ag-Te alloy electrode migrates over to the Ag electrode, resulting in decreased activity of Ag in the Ag-Te alloy, and thus an increased potential difference between both electrodes. The inventors of this device termed this state of operation as "charging". When the polarity of the applied DC voltage is reversed to that of the former case, Ag is refilled into the Ag-Te alloy, resulting in the decreased potential difference and returns to the initial value eventually. The inventors of this device termed this state of operating as "discharging". Study disclosed by the inventors of this device indicates that the electromotive force generated by the abovementioned charging or discharging current can cause a linear change to some extent with respect to the charging or discharging time. Thus this device makes it possible to effect write-in and non-destructive read-out operation while preserving relatively linear relation between the charging or discharging time and terminal voltage, and in addition, it can hold the memory condition for a relatively longer period of time. The referenced patent further discloses an improved electrochemical potential memory device. More specifically, FIG. 6 of the referenced patent shows an improved electrochemical potential memory device for eliminating the IR drop across the resistance in the electrolyte and the overvoltage caused by dissolution or deposition of Ag, which is basically characterized by the provision of an auxiliary cathode that comprises an output terminal for detecting the potential separately from the abovementioned cathode available for the input terminal for the current conduction. In view of these advantageous characteristics of the abovementioned memory device, it is possible to utilize this device as a voltage source for providing a voltage variable in response to manual operation to a voltage controlled variable reactance device employed in an automatic tuning apparatus.
As described previously, a typical prior art automatic tuning apparatus comprises a capacitor connected to provide a terminal voltage thereof to a voltage controlled variable capacitance diode coupled associated with a tuning circuit of the automatic tuning apparatus, and a charging/discharging control circuit operable in response to manual operation for enabling the control circuit for charging or discharging the capacitor and in response to an output of said tuning circuit for disabling the control circuit to discontinue to charge or discharge the capacitor, so that the voltage thus provided across the capacitor is applied to the variable capacitance diode to cause a desired capacitance in the diode with which the tuning circuit is tuned to a desired frequency to provide a tuning output therefrom, which tuning output is applied to the charging/discharging control circuit, as described above. In such an automatic tuning apparatus, now consider a case where a power supply is turned off after a given broadcast is once received as a result of tuning thereto and the power supply is turned on again thereafter. In such a situation it is desired that the same broadcast wave received before the power supply was turned off is received again even after the power supply is turned on. Nevertheless, any charge stored in the capacitor is discharged after the power supply is turned off, resulting in a change in the terminal voltage of the capacitor. Thus, the same broadcast wave as received before is not necessarily received when the power supply is turned on again.
The above described inconvenience by adoption of a capacitor as a voltage source for providing a varying voltage can be solved to some extent by adoption of the above described solid state potential memory device, inasmuch as the above described potential memory device has a characteristic being capable of holding the terminal voltage thereof for so many hours. However, detuning could happen because of a drift in the above described potential memory device, a direct current amplifier for amplifying the terminal voltage of the device, the tuner, etc. caused by variation of the ambient temperature and fluctuation of the voltage source. Such detuning could be solved by provision of an automatic frequency control. Nevertheless, adoption of the automatic frequency control in the automatic tuning apparatus can not solve any inconveniencies described previously which are encountered when the power supply is turned on again after the tuner is tuned to a given broadcast wave and the power supply is turned off.